1. Field of the Invention
This invention relates to an image forming device and its deviation correcting method, and more particularly to a tandem type color image forming device (such as color PPC, MFP, fax or printer) which controls individually the rotational speeds of motors (such as brushless motors) which rotate a plurality of image carriers, to correct a deviation (color deviation), and its deviation correcting method.
2. Description of the Related Art
Among electrophotographic image forming devices, the number of color image forming devices, such as color copiers and color printers, are increasing. Especially, the number of tandem-type color image forming devices are increasing. In such image forming device, a plurality of photoconductors are arranged along with a plurality of developing devices respectively, a monochrome toner image is formed on each photoconductor, respectively, and those monochrome toner images are transferred one by one to a sheet recording medium so that a color image is recorded on the sheet recording medium.
FIG. 9A, FIG. 9B and FIG. 9C are diagrams showing the composition of imaging units of some image forming devices according to the related art.
The image forming device of FIG. 9A is a tandem type electrophotographic color image forming device in which a plurality of photoconductors 11Y, 11C, 11M, and 11B (collectively called photoconductor 11) are arranged along with a plurality of developing devices 12Y, 12C, 12M, and 12B (collectively called developing device 12) and a plurality of transferring devices 13Y, 13C, 13M, and 13B (collectively called transferring device 13), respectively.
In the image forming device of FIG. 9A, a monochromatic toner image is formed on the surface of each photoconductor 11, respectively, and those monochrome toner images sequentially come in contact with an intermediate transfer belt 15 to form a combined color image on the intermediate transfer belt 15. The combined color image is transferred to a sheet recording medium so that a full color image can be formed.
In the image forming device of FIG. 9A, the plurality of drum-shaped photoconductors 11Y, 11C, 11M, and 11B are installed side by side. Each photoconductor is an independently rotatable image carrier, and an image of a different color is formed on the surface of each photoconductor, respectively.
Each image formed on each photoconductor 11 is transferred to the intermediate transfer belt 15 at a transfer position corresponding to each photoconductor 11, respectively. At the respective transfer positions on the intermediate transfer belt 15, the transferring devices 13Y, 13C, 13M, and 13B are moved up and down and those images on the respective photoconductors 11 are transferred.
By operating the contact/separation mechanisms 14YMC and 14B, the transferring devices 13 are moved up or down, so that they are brought in contact with or separated from the intermediate transfer belt 15. Around the periphery of each photoconductor 11, the developing device 12, the charging device 16, the cleaning device 17, and the discharging device 18 are arranged. Each photoconductor 11 is scanned by a laser beam from the laser writing unit 19 in accordance with the image signal of each color and an electrostatic latent image is formed on the photoconductor 11.
FIG. 9B shows the composition of the imaging unit of a direct-transfer type image forming device. In the direct transfer type, an image on a photoconductor is directly transferred to a recording medium.
In FIG. 9B, the elements which are the same as corresponding elements in FIG. 9A are designated by the same reference numerals and a description thereof will be omitted. In the image forming device of FIG. 9B, there are provided the photoconductors 11Y, 11C, 11M and 11B. Each photoconductor is an independently rotatable image carrier, and an image of a different color is formed thereon, respectively.
The images formed on the respective photoconductors 11 are directly transferred to a recording paper P by the transferring devices 13 at their corresponding transfer positions, respectively. The transferring devices 13Y, 13C, 13M, and 13B are moved up and down to the transfer conveying belt 30 at the transfer positions. By operating the contact/separation mechanisms 14YMC and 14B, the transferring devices 13 are brought in contact with or separated from the transfer conveying belt 30.
The transfer conveying belt 30 is stretched and wound between the driving roller 32 rotated by the transport motor 31 and the follower roller 33, and the direction of the rotation of the transfer conveying belt 30 is indicated by the arrow in FIG. 9B.
FIG. 9C shows the composition of the imaging unit of a tandem type image forming device having first and second intermediate transfer mediums. In this image forming device, there are provided the photoconductors 11Y, 11C, 11M, and 11B. Each photoconductor is an independently rotatable image carrier, and images of yellow, cyan, magenta, and black are formed on the photoconductors 11, respectively. The images formed on the two photoconductors 11Y and 11C among the four photoconductors are transferred to the intermediate transfer medium 34A at the primary transfer position P5 and P6.
The images formed on the remaining two photoconductors 11M and 11B are transferred to the intermediate transfer medium 34B at the primary transfer positions P7 and P8.
The intermediate transfer mediums 34A and 34B are rotatable independently. The intermediate transfer mediums 34A and 34B are rotated by the first medium transfer motors 35A and 35B. There are provided the transferring devices 13Y, 13C, 13M, and 13B and the contact/separation mechanisms 14Y, 14C, 14M, and 14B (collectively called contact/separation mechanism 14).
A monochromatic toner image is formed on each photoconductor 11, respectively. The contact/separation mechanism 14 is operated to bring the transferring device 13 in contact with the first intermediate transfer belt. The monochrome toner images are transferred one by one to the first intermediate transfer belt.
This image forming device includes an intermediate transfer drum 36 to which the images from the intermediate transfer mediums 34A and 34B are transferred respectively at the secondary transfer positions P9 and P10. The intermediate transfer drum 36 is rotated by the second medium transfer motor 37.
The image transferred by intermediate transfer drum 36 is transferred to the recording paper P with transfer roller 38 at the third transfer position P11. There is provided the transport belt 39 which is rotated in the direction to convey the recording paper P, and the transport belt 39 is stretched and wound between the driving roller 40 and the follower roller 41. When the driving roller 40 is rotated by the motor 42, the transport belt 39 is rotated in the direction indicated by the arrow in FIG. 9C.
In a color image forming device, the position where color images are combined may be shifted from the right position, and a color deviation or the like may occur on the combined image. Among the causes, there are the deviation of the laser beam irradiation angle when forming a latent image, the roughness of the timing resolution of an optical writing unit, the deviation of the mounting positions of photoconductor units, etc.
When the toner images of respective colors are transferred, a deviation (color deviation) may occur under those influences. If the transfer position is shifted from the desired transfer position, color irregularity or color deviation may occur on the combined image as a result of transferring and combining the images of colors.
In order to solve the above problem, it is necessary to reduce deviation (color deviation) by arranging a deviation correction unit that changes the rotational speed of each photoconductor and adjusts finely the time from exposure to transfer.
For example, Japanese Laid-Open Patent Application No. 2006-047990 discloses an image forming device which corrects deviation (color deviation) by changing the rotational speed of the motor which rotates each photoconductor, and changing the image resist position in the transfer unit.
The image forming device of Japanese Laid-Open Patent Application No. 2006-047990 uses a deviation correcting method. In this method, the rotational speed of each image carrier is adjusted so that the time for the image formed in the exposure position on each image carrier arrive at the transfer position is set to the same value between the image carriers when transferring the respective images formed on the plurality of image carriers to the transferring medium to form a combined color image.
In the image forming device of Japanese Laid-Open Patent Application No. 2006-047990, in advance of image formation, a pattern for detection of color deviation is formed on the transferring medium, and the amount of color deviation is determined based on the result of detection of the pattern for color deviation detection using the sensor.
However, to change actually the rotational speed of the motor rotating the image carrier in order to correct the detected color deviation, a control mode for color deviation correction must be set up in the image forming device. During this control mode, operation of the motor must be suspended so that the rotational speed of the motor can be changed.
For this reason, in order to form a color image after the correction of color deviation, an excessively long time which includes the time to perform the control mode for color deviation correction, and the time to restart the motor and reach a target rotational speed, in addition to the time for image formation, was required according to the related art.